Radio air raid warning system



March 21, 1944. w` R, KOCH 2,344,618 i RADIO AIR RAID WARNING SYSTEM Filed July 3l, 1:941

#scf/me Z5 :All MMM* L@ Patented Mar. 21, 1944 RADIO AIR RAID WARNING SYSTEM Winnen n. noch, naaaoneeia, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application July 31, 1941, serial No. ite-'9oz (ci. 25e-2o) 6 Claims.

This invention relates to a radio air raid warning system, responsive to one or more subaudible control signals to reproduce intelligence transmitted on a carrier or mean frequency signal wave, and responsive to one or other subaudible control signals to restore the system to another condition of operation.

More particularly, this invention relates to a radio signal receiving system which is rendered responsive or non-responsive to broadcast signals or other intelligence transmitted lloy radio wave, when the radio Wave is modulated by predetermined subaudible control signals preceding and following the intelligence transmission` and has for its object to provide a receiving system of the character referred to, embodying an improved and simplified tone or control signal selection network.

In tone controlled radio systems, a series of vibrating reed relays responsive to differing control frequencies are most frequently provided as the frequency selecting and control means. However, in accordance with the present invention, such relays and the fundamental disadvantages associatedy with their use, may be eliminated by substituting electrical circuits, includ'- ing resistance-capacity networks in connection with suitable amplifiers, and in such a manner that a high degree of selectivity to control signals and greater reliability is achieved.

It is a further object of this invention, therefore, to provide an improved control arrangement for radio air raid Warning systems and the like, wherein a plurality of parallel amplifier channels are provided, one for each tone or control frequency, embodying resistance-capacity networks in a plurality of stages, and with regenerative feedback for improving the selectivity characteristic of the system, and inverse or degenerative feedback fo-r stabilizing the amplifier operation when utilizing a high degree of regenerative feedback.

With this arrangement, electrical, instead of mechanical, tone selection is made possible with a reduction in cost, in addition to simplifying the system.

The invention will, however, be further understood from the following description, when considered in connection with the accompanying drawing, and its scope is pointed out in the appended claims.

In the drawing the figure is a schematic circuit diagram of a signal receiving system provided' with subaudible frequency control means embodying the invention.

Referring to the drawing, a source of lsignal energy is indicated by the antenna circuit 5 in which is located a coupling winding B associated with a signal input loop o r inductance 'l for a detector-oscillator or mixer stage the tunable oscillator circuit of which is indicated atS'. The input and the oscillator circuits are provided with tuning elements It! and Il of any suitable type such as variable capacitors as shown, preferably jointly operable, as indicated by the dottedco'nnectiori I2, for tuning the system to selected signals in a predetermined frequency band.

The intermediate frequency output circuit I4 of the mixer stage is coupled through an intermediate frequency coupling transformer |f5 to an intermediate frequency amplifier stage I6, which' is' coupled through a similar transformer ll ,to a second or audio frequency detector' It. VThe latter comprises a diode rectifier having anode electrodes l`9 and a cathode 20, connected with a .suitableV output impedance 2 It.

The detector IB further includes an audio frequency amplifier stage comprising a control grid 25 and an anode 26r associated with the cathode 20. The grid is coupled through a' variable volume control contact 2l with the detector output resistor 2l.

The output circuit 28 from the'anode is coupled to a suitable amplifier comprising an output amplifier stage 30, and the latter is coupled through an output transformer 3l, with a loud"-l speaker device 32.

The receiving system proper may also include AVC means, which in simplified 4form comprises f the second detector and the output impedance 2|. In the present case, the D.,C; component of the rectified signal from theim'pedance 2l is applied to the AVC gain control circuit ,35 for the mixer and I.-`F. amplifier, througha suit'- able filter comprising a series resistor and a shunt filter capacitor 3l.

The system thus far described represents' any suitable radio signal receiving system. However;

in the case of a standby receiver system for air raid signals and the' like, a minimum number of tubes is desirable, at least in the signal receiving portion thereof, to' conserve pbwer'a'nd torduce operating cost as it is necessarily maintained energized for the reception of signals to r which the system is tuned.

In any case,` as inV the present example, the" receiving system is preferably' readily tunable to a desired signal channel or .broadcasting station from which" sighalsarekdesired or' eii'p'cteld, arid includes automatic volume control means for maintaining the signal intensity at the audio frequency detector substantially constant, and volume control means for adjusting the output signal level. The system operates to amplify, detect and reproduce a selected signal comprising a modulated carrier wave or mean frequency signal.

The mixer, intermediate frequency amplifier and audio frequency detector, which may be designated as the signal receiving portion of the system, must be energized whenever signals are to be received, which may include continuous operation over extended periods of timefor air raid signal reception, while the audio frequency of power amplifying portion 'of the system may be deenergized in any suitable manner to conserve power and to increase the tube life. Also, to prevent noise and undesired signa1 reception, the sound output may be cut oi in the absence of desired signals, and both control functions, that is, power conservation and the reduction of the sound output may be provided by deenergizing the cathode or heater circuit of the audio frequency amplifying system, and particularly the power output stage heater circuit. For this purpose, in the present example, the output or power amplifier cathode heater, indicated at 40, is normally deenergized, whereby signals to the loudspeaker 32 and the heavy anode current drain of the output stage or audio frequency amplifier are cut off in the absence of desired signals.

The output stage in the present example and the loudspeaker operation are controlled by subaudible modulation frequency signals applied to the carrier wave, for example, a 24 cycle tone or control frequency is transmitted on the carrier for a few seconds just preceding important announcements or progra-ms, and a 36 cycle tone or other subaudible control frequency signal yis transmitted on the carrier just after the announcement or program. The 24 cycle control frequency signal causes the receiving system to be energized. In the present example, this comprises energizing the filament or heater 40 of the output stage. The 36 cycle control frequency signal deenergizes the receiver, as in the present example, by causing the filament or heater 40 to 'be deenergized.

The sub-audible control frequency signals may be derived from the receiving portion of the system at any suitable point preceding the deenergizable amplifying portion, such as at the high potential output terminal 42 of the second or audio frequency detector output impedance 2|, and the signals so derived are utilized to operate a single relay device 43 which may be of simple conventional design, as distinguished from the usual number of tuned or vibrating reed relays normally used in systems of this character for control purposes.

In accordance with the invention, the relay operating winding 45 is connected in or otherwise coupled to the output circuits 46 and 41 of two parallel control channels 48 and 49, respectively, the input circuits 50 and 5I of which are connected with the subaudible signal supply terminal 42 through a supply lead 52.

The lead 52 includes a low pass filter comprising a series resistor 53 and a shunt bypass capacitor 54 for cutting oil' the audio frequency Sound signals.l The input circuits 50 and 5l also include suitable high resistance decoupling resistors connected at 55 and 56 which prevent the amplifying channels 48 and 49 from appreciably loading the detector output impedance 2l. These may each have a resistance of several megohms to prevent each selective amplifier from affecting the other.

Each of the amplifying channels 48 and 49 comprise a two stage amplifier and detector coupled by means of resistance-capacity networks. In the amplifying channel 48 the first stage amplifier is indicated at 51 and the second stage amplifier at 58. The first stage input grid 59 is connected through a lead 88 with the output terminal El of a resistance-capacity network 62 the input terminal 63 of which is connected with the anode 64 of the first stage to provide regenerative coupling lbetween the anode and grid circuits. The cathode E5 is connected to ground which is the negative anode supply terminal through an unbypassed cathode resistor S1.

In the ladder type network 62, the signal input terminal for the network indicated at 68, is connected with the input circuit 58, whereby one resistor element 69 forms part of the input circuit as well as one of the elements of the network, The terminal 68 is next adjacent to the output terminal 6I so that the incoming subaudi'ble frequency signal is applied to the control grid 59 through part of the network 62. The feedback input terminal 63 of the network connected with the anode 64 is 180 out-of-phase with the output or grid terminal 6l so that regenerative feedback is applied to the grid 59 along with the incoming signal at a predetermined subaudible frequency.

The regeneration is prevented from extending into the oscillating range by means of degenerative feedback introduced into the input circuit through the unbypassed cathode resistor 61 which is of relatively high resistance to provide a high degree of negative or inverse feedback. A resistor 10 is also provided in the grid circuit 6D to prevent overload of the amplifier stage on noise pulses and to aid in preventing oscillations in the amplifier stage.

The filter network 62 providing 180 phase shift between the anode or input terminal and the output or grid terminal comprises a ladder circuit of shunt resistors 15 and the signal input circuit resistor 69 connected to ground, an anode coupling resistor 16 in the output circuit of the amplifier 51, and a bias supply resistor 11 connected with the output terminal 6I and the control grid 59. The resistor 11 is connected with a bias potential supply lead 18 which receives from a terminal 19 a potential which is positive with respect to ground 88 and the cathode, through a bleeder resistor comprising sections 8| and 82 in series. This is in opposition to the negative potential provided by the self bias re` sistor 61 and reduces the latter potential to a normal value for establishing a high gain through the stage.

The series elements of the network comprise capacitors interposed between the resistor elements. Thus it will be noted that the various circuits-associated with the resistor elements are effectively isolated for D.C. potentials. This arrangement permits different D.C. operating potentials to be applied to the various tube electrodes without D.C. inter-coupling, while the desired signal or alternating current path is maintained through the filter network.

The output circuit of thevstage 51 is coupled to the succeeding stage 58 through a circuit lead 81 connected through a suitable decoupling resistor agili-4,618

84y withV a' terminal 83 in the network preferably neirtisajacent to the anode `or input terrmnai s3. The second `stage amplifier coupling network 88 is essentially the same as the network B2, comprising series capacitor elements 89 and shunt resistor elements 99. An anode coupling resistor 9| inthe network is connected with the anode output circuit 92 at the network input terminal 93, and a grid bias supply resistor 94 is connected with the output terminal 95 of the filter network, which terminal is also connected through a lead 98` with the control grid 91 of the amplier'stage '8. I The cathode 99 is likewise connected to ground through an unbypassed cathode resistor |99.

Itwill be noted that the signal inputl lead 81 is connected with a signal input terminal I`|l`| next adjacent to the feedback output terminal 95` of the network and that the anode or feedback input terminal 93 is coupled to the signal output terrinal |94l next adjacent thereto 'in the network, asV inV the case of the preceding stage.

The signal output tube of the amplifying channel 48 isa detector |95 comprising a pair of parallel connected anodes ||lv|5v and a cathode |91, the latter being connected through a lead |98 and relay contacts |99 and IIO with ground III. This provides a connection with the ground terminal o f the first grounded resistor 99 of the network 88, which .is the detector output impedance, as well as one of the coupling elements of the netw'ork.

The output circuit of the amplifying channel 48 is the output anode circuit of an amplifier included in the tube |95 with the rectifier, and f having the' cathode |91 in common therewith. The output anode I I5 is connected with the output circuit 46 and the relay winding 45 and receives energy from a positive anode potential supply lead indicated at H6'.

The control grid of the amplifier is indicated at ||1 and is connected with the output impedance of the detector through a lead I I8 and the terminal |94. A filter comprising a series resistor |29 and a bypass capacitor |2I is provided in the connection to prevent the application to the grid |l|1 of any signal component and only the D.C. component of the rectified subaudible signal frequency.

The amplifying channel 48 thus comprises a two stage resistance-capacity network in circuit with each of two amplifiers cascade-connected between the control signal supply terminal 42 and the rectifier or detector I95'-`I96. Each network includes a suicient number of successive ele@ ments withV the proper resistance and capacity values to effect an 189 phase shift between the input or anode end of each network and the output or grid end thereof. In other Words, each network, together with its associated amplifier tube, is regenerative at a frequency which gives 180 phase shift through the network between anode and control grid of the stage, and at this frequency the network circuit becomes sharply selective. In the present example, this frequency for the channel 48 is chosen at 24 cycles per s'ec-l ond which is one of the control signal frequencies. The large negative feedback provided by the unbypassed cathode resistors 61 and |99 prevent the amplifier network from oscillating while maintaining the selectivity provided by the regeneration effect.

`The' cathodes of the signal receiving portion of the system and' ofthe amplifier and detector .stages of the 24 cycle control frequency amplify'-4 power or operating switch indicated at |34.

' circuitin'dicate'd atA |25. In this circuit the 24 cycle amplifier filaments are indicated at |26 and the receiver laments at |21, being connected with the grounded side |28 of power supply circuit terminals |29 for alternating or direct current. The circuit |25 also includes a current limiting resistor |39 and the cathode heater or iilament |3| of a power rectier tube |32 which is connected' with the high potential side of the supply terminals |29 through a lead |33 and a A pilot lamp; |35 for the receiver is connected across a portion of the rectiiier filament I3 I. The cathode |38A of the rectifier |32 is also connected with the high potential supply lead |33, while the.

anode |31 is connected through a suitable filter |38` with the positive anode supply lead |I6 for the receiver and the amplifiers. `The various electrodal connections with this lead for the various tubes in the system have been omitted for the sake of simplifying the drawing.

The power supply and cathode heater circuit thus far described is of the so-called A. C.D. C. type which permits the supply circuit terminals |29 to be connected with any suitable alternating z or direct current supply source of the proper voltage.

The amplifying channel 48 is essentially the same as the amplifying channel 48 except that it is made responsive to a different subaudible control frequency, such as 36 cycles, for example, Aand comprises a first stage amplifier |49 provided with a 180 phase shift network I4| between the anode circuit |42 and the grid circuit |43 which causes the amplifier to be regenerative at 35 cycles.v The amplifier is stabilized by the inverse or degenerative feedback introduced through the unbypassed cathode resistor |44.

Control signals from the terminal 42 of the receiving system are applied to the signal input r' terminal |45 of the network adjacent to the grid terminal |45 and are derived from the amplifier stage through the terminal |41 adjacent to the anode or feedback input terminal |48 The output coupling connection is provided through a lead |59 whichis connected with la signal input terminal |5| for the regenerative coupling network |52V of the succeeding or second stage ampliiierA |53. This amplifier is connected in the same manner as the iirst sta-ge and as in the amplifying channel 48.

The control signal output terminal is indicated at |54 and is connected with a control grid |55 of a detector |56 which forms the terminal tube of the amplifying channel 49, corresponding to the detector of the channel 48. In this case the detector is of the triode type having a, Cath.' ode resistor |51 coupled to the control grid I 58 of a direct current amplifier |59 through 'a filter resistor |69 for which a filter capacitor ISI is provided, thereby to apply to the grid |58 only the direct current component of the rectified subaudible control frequency sign-a1 transmitted through the channel 49. The amplifier output circuit is connected with the lead 41 so that output current variations serve to control the relay 45 along or in parallel with the control provided through the output circuit 46 of the channel 49.

The heaters or laments for the tubes |49, |53, |56 and |5901' the 36 cycle channel amplifier are indicated at |65 and are connected on one side other side with the high potential supply lead |33 of the power supply circuit through a, lead |61,

the cathode heater 40 of the output ampliier stage 36 and a current limiting resistor |68.

When the relay 45 is deenergized the contact ||0 is caused to close with the contact |66 therebyV plifier channels 48 and 49, from the rectifier |32, l

as its heater and the receiver and 24 cycle amplier heaters |26 and |21 are energized. Assuming the relay contacts |09 and ||0 to` be closed initially to energize the relay winding 45, in the absence of a received signal, the grid II'I of the detector |85 in the amplifier channel 4B is substantially without bias and the anode current through the circuit 45 and the relay operating winding 45 is maximum, causing the relay contacts to be held closed in the position shown.

When a 24 cycle signal is received on a carrier wave to which the receiver is tuned, the 24 cycle signal is applied to both amplier channels and is amplied in the channel 48 causing a negative bias to be applied to the grid H1 from the rectifier output resistor 98 in the input end of the network 88 and a reduction in the anode current through the circuit 45 and the winding 45, thereby causing the relay contacts |09 and I ID to open rendering tube |85 inoperative, and permitting the contact HB to close with the contact |66. This energizes the 36 cycle amplifying channel, that is, the cathode heating circuit, in the present example, which is then in readiness to receive and amplify a 36 cycle signal on the same carrier when the transmission of intelligence over the receiving system is completed. The receiver is put into operation by the heater 48 of the output stage 30 being energized along with the heaters of the 36 cycle amplifier and while energized, signals may be received, such as air-raid warnings or other intelligence as desired on the same or any carrier wave to which the receiver is tuned.

Upon completion of the reception desired, the receiver is deenergized or rendered inoperative to receive signals upon reception of a 36 cycle signal, which is sharply amplified in the amplifier 49 and causes the detector grid |55 to become l more positive. The increase in anode current through the cathode resistor |51 causes a positive bias to be applied to the grid |58 thereby increasing the anode current through the circuit 4l and the relay winding 45 which'moves the contacts into the position shown whereupon the current through the anode circuit 46 continues to hold the contacts |99 and ||0 closed. 'I'.his the system is again in readiness to receive a 24 cycle controlling signal, while the output is silenced since the 36 cycle amplifier and output amplifier cathode heaters are deenergized. A considerable saving in operating current is provided by thus causing the 36 cycle amplifier heat,- ers to be deenergized along with the power output heater 49. Y

By staggering the frequency of maximum response of the networks 88 and 89 and the networks |52 and |4| the resonant frequency of the signal selecting amplifiers may provide an overall with a contact |66 of the relay 43 and on theV wider or broad band pass selectivity characteristic so that the system may readily respond to the control signals although the tuning at the receiver or at the transmitter may shift slightly. By providing separate stages or a plurality of networks in the separate amplifying channels, a low value of equivalent selectivity with better discrimination against operation by noise or interference is provided thanv by the use of the customary tuned vibratory relaymeans.

Furthermore, as hereinbefore pointed out, the resistor-capacity networks are relatively simple and of low cost design while permitting the application of D.-C. operating potentials to the various electrodes, eachcircuit being independent of the other, and byv utilizing a suflcient number of elements in the networks the same may be made regenerativeV at a frequency which gives phase shift, whichV frequency is that for which the ampliiier is tuned. Hence the highly regenerative circuit may be made sharply responsive to any desired signal or control frequency While being stabilized against oscillation by the degeneration introduced in each stage by the large cathoderesistors without bypass.

I claim as my invention:

l. The combination with a radio signal receiving system, of a plurality of sub-audible control frequency amplifying and detecting channels each including a detector and a preceding amplifier stage having a control grid circuit and an anode circuit, a resistance-capacity interstage coupling network connected between said grid and anode circuits in each channel providing a 180 phase shift at predetermined differing sub-audible control frequencies, whereby said control channels are regenerative and selective at said frequencies, a stand-by signal receiving portion including a signal detector, means for deriving said subaudible signals from said last named detector, means for applying said signals at said sub-audible frequencies to said control channels, and means responsive to the detector output of each amplier channel .for controlling a condition of l network connected between said grid and anode` circuits in each channel providing a 180 phase shift at predetermined diiering sub-audible control frequencies, whereby said control channels are regenerative and selective at said frequencies, a stand-by signal receiving portion including a signal detector, means for deriving said subaudible signals from said last named detector, means for applying said signals at said sub-audible frequencies to said control channels, and means connected with the detector output of each amplifier channel for rendering said system operative'for normal intelligence transmission in respense to one sub-audible signal and for rendering said system inoperative for normal intelligence transmission in response to the other of said subaudible signals.

3. In a radio signal receiving system, a plurality of control channels each including an amplier tube having a control grid circuit and an anode circuit, a resistance-capacity network connected between said grid and anode circuits providing regenerative and selective response therein at dierent subaudible control frequencies. means comprising a stand-by signal receiving channel for applying to said amplifiers modulated signals including said frequencies, a detector connected with the output of each of said amplifiers, and means responsive to the detector output of each amplifier channel for controlling a different condition of operation of said receiving system.

4. The combination with a signal receiving system, of means for deriving therefrom sub-audible frequency control signals, a pair of parallel connected anipliers each comprising an amplier stage having a control grid circuit and an output anode circuit, a resistance-capacity interstage coupling network connected between said circuits in each of said amplifiers providing successive resistance and capacity elements to effect a 180 phase shift between said circuits at predetermined sub-audible control frequencies, whereby said ampliers are regenerative and selective at said frequencies, means in each of said amplifiers for establishing a predetermined degree of inverse stabilizing feedback, whereby said amplifiers are selectively responsive to a modulated signal wave when said wave is modulated by predetermined sub-audible control signals preceding and following intelligence transmission thereon, means for applying said control signals at said frequencies to said amplifiers, and means for deriving from said amplifiers a control current which changes in magnitude and direction in response to each of said control signals.

5. The combination with a signal receiving system, of a circuit for energizing at least a portion of said receiving system, relay means controlling said circuit, a pair of amplifying channels each having an output circuit connected with said relay means to apply operating current thereto, and including an amplifier stage having a control grid circuit and an output anode circuit, a resistance-capacity interstage coupling network connected between said circuits providing a 180 phase shift at predetermined sub-audible control frequencies, whereby said amplifier stages are regenerative at said frequencies, means in said amplifier stages for establishing a predetermined degree of inverse stabilizing feedback, whereby said amplifier channels are selectively responsive to a modulated signal wave when said wave is modulated by predetermined sub-audible control signals preceding and following intelligence transmission thereon for controlling said relay means, means for applying said control signals at said frequencies to said amplifying channels, and means for deriving from said amplifying channels a control current which changes in magnitude and direction in response to each of said control signals.

6. The combination with a signal receiving system, of means for rendering said system responive to a plurality of sub-audible control signals to reproduce intelligence transmitted on' a received carrier wave and for rendering said system non-responsive in the reproduction of intelligence selectively, said means including a circuit for energizing at least a portion of said receiving system through which signals are translated, a relay controlling said circuit, a pair of amplifying channels each having an output circuit connected with said relay to apply operating current thereto, and including an amplifier stage having a control grid circuit and an output anode circuit. a resistance-capacity interstage coupling network connected between said circuits in each of said ampliiier channels providing successive resistance and capacity elements to effect a 180 phase shift between said circuits at predetermined sub-audible control frequencies, whereby said amplier channels are regenerative at said frequencies, means in each of said amplifier channels for establishing a predetermined degree of inverse stabilizing feedback, whereby said amplitier channels are selectively responsive to a modulated signal wave when said wave is modulated by predetermined sub-audible control signals preceding and following intelligence transmission thereon for controlling said relay, means for applying said control signals at said frequencies to said amplier channels, and means for deriving from said amplier channels a control current which changes in magnitude and direction in response to each of said control signals.

WINF'IELD R. KOCH. 

